My impression is that most/all teleconverters will vignette at the wide angle end of the zoom.

This is another case illustrating why using X times multiplier numbers to describe the range of a lens's focal length is a bit silly. The Fuji S3500 zoom range is 39-234mm (equiv). A 1.7 teleconverter will make the top end of that range 398mm, but the bottom end will be determined by the point vignetting sets in: it is very unlikey to go all the way down to 66mm. If it vignettes at something like 300mm, the range 235-300mm will be unusable.

Regardless of vignetting, a teleconverter will not increase the range of the lens while it is attached. You will be able to cover 39mm to 398mm only by removing the teleconverter to get the lower end of the range.

Any brand xD card will work. There are only two camera lines (Fuji and Olympus) that use this format. I just did some searching for a friend and found a 512MB card on NewEgg for about $80 plus $5.00 shipping.